The present disclosure relates to electrochemical cells, and in particular to regenerative fuel cell systems.
Referring to prior art FIG. 1, a partial section of a typical proton exchange membrane fuel cell 100 is detailed. In fuel cell 100, hydrogen gas 112 and reactant water 114 are introduced to a hydrogen electrode (anode) 116, while oxygen gas 118 is introduced to an oxygen electrode (cathode) 120. The hydrogen gas 112 for fuel cell operation can originate from a pure hydrogen source, methanol, or other hydrogen source. Hydrogen gas electrochemically reacts at anode 116 to produce hydrogen ions (protons) and electrons, wherein the electrons flow from anode 116 through an electrically connected external load 121, and the protons migrate through a membrane 122 to cathode 120. At cathode 120, the protons and electrons react with the oxygen gas to form resultant water 114′, which additionally includes any reactant water 114 dragged through membrane 122 to cathode 120. The electrical potential across anode 116 and cathode 120 can be exploited to power an external load.
The same configuration as is depicted in FIG. 1 for a fuel cell can be employed for electrolysis cells. In a typical anode feed water electrolysis cell (not shown), process water is fed into a cell on the side of the oxygen electrode (in an electrolytic cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolytic cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell (not shown), process water is fed on the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the process water exits the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.
The typical electrochemical cell includes one or more individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. In certain conventional arrangements, the anode, cathode, or both are gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly”, or “MEA”) is typically supported on both sides by flow fields comprising screen packs or bipolar plates. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA. Since a differential pressure often exists in the cells, compression pads or other compression means are often employed to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems.
A typical regenerative fuel system generally includes an electrolyzer stack in fluid communication with a fuel cell stack or it may include a reversible electrolyzer/fuel cell stack. In the electrolysis mode, i.e., charging mode, electrical power supplies energy to the electrolyzer to produce hydrogen gas by electrolyzing water, which may then be stored or used in the fuel cell. In the fuel-cell mode, i.e., discharge mode, the stored hydrogen is combined with air to generate electrical power and water. The water is then recycled back to a water storage vessel. During use of the regenerative fuel system, there may be periods where the fuel cell is operated for prolonged periods of time. Operating in the fuel cell mode for prolonged periods of time may use hydrogen supplied from other than the electrolysis cell and may produce excess by-product water. Since the size of the water storage vessel employed in regenerative fuel systems is generally limited, excess production of water during fuel cell operation can reach the maximum capacity of the vessel.